1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses of large relative aperture, while still having good optical performance over the entire zooming range, suited to photographic cameras or video cameras.
2. Description of the Related Art
In photographic cameras or video cameras, there has been demand for zoom lenses of large relative aperture and high range with high optical performance.
Of these, the video camera, because its image sensor is relatively low in sensitivity, requires that the relative aperture of the zoom lens is as large as possible.
Beside this, in the case of the color video camera using the stripe filter in front of the single-tube type color image sensing tube or the image sensing plate such as CCD, for the purpose of preventing color mixture, it is required that the whole optical system be constructed in telecentric form so that all rays of light which are to enter the stripe filter become as normal thereto as possible.
Further, as the CCD, MOS or like image sensors are widely used at present in the video cameras, the surface of its cover glass and the surface of that image sensor generally have high reflectances. The light reflected from these surfaces, therefore, reflects from the lens surfaces of the photographic lens and the inside of the lens barrel and enters again the image sensor thus becoming a cause of producing the so-called ghost or flare.
In this regard, a technique of reducing that ghost and flare that arises from the reflection from the image sensor by specifying the lens surfaces of the relay lens unit under a certain condition is proposed in Japanese Laid-Open patent application No. Sho 57-147612 and U.S. patent application Ser. No. 223,108 filed on July 22, 1988.
By the way, the video camera, etc. is included also with the diaphragm and a filter member such as ND filter located usually in that space of the photographic lens which forms an afocal optical system. The reflected light from this filter enters again the image sensor, forming a ghost image thereon. A technique of preventing this from occurring is proposed in U.S. patent application Ser. No. 301,951 filed on Jan. 26, 1989 now U.S. Pat. No. 4,892,397.
Meanwhile, as for the zoom lenses suited to the video cameras, mention may be made of those disclosed in U.S. Pat. Nos. 4,618,219, 4,621,905, 4,659,187, 4,653,874 and 4,518,228, Japanese Laid-Open patent application No. Sho 59-222807, Japanese Laid-Open patent application No. Sho 60-260912 and Japanese Laid-Open patent application No. Sho 61-20291, and U.S. Pat. No. 4,812,024.